As described in U.S. Pat. No. 4,486,709, depth pulse schemes generate signals from sample regions where the excitation pulse .theta. is close to m.pi./2, where m is an odd integer. However signals from regions where .theta.=3.pi./2 or 5.pi./2, if such regions exist, can be eliminated by varying some of the pulses from their ideal .theta. or 2.theta. values. The sample discrimination afforded by means of these pulse schemes may be improved by using static or pulsed magnetic field gradients or by using more than one radiofrequency coil.
This invention relates to the application of specific radiofrequency pulse schemes to inhomogeneous irradiation coils in a nuclear magnetic resonance (NMR) method of analysis, so that only signals from specific sample regions may be detected and so that some multipulse NMR methods may be achieved.
The terminology used in this disclosure is commonly used in the NMR literature.
In an NMR experiment using a single excitation pulse, where this pulse is applied to a coil which provides a grossly inhomogeneous radiofrequency field, the pulse angle varies widely across the sample. If .theta. is the pulse angle at a point in the sample, the nuclear magnetization component, which is rotated in the transverse plane of the rotating plane by the pulse, is proportional to sin .theta. and the signal detected using the coil is proportional to .theta. sin .theta.. Thus the signal intensity from different parts of the sample varies widely. Furthermore, although there is some sample discrimination, in that some regions provide much more signal than others, the signal intensity is only close to zero from small regions where the .theta. pulse angle is close to n.pi. (n=0, 1, 2 . . . ). For example, one method for obtaining a high resolution NMR spectrum from a selected portion of a larger sample, which has gained recent popularity, involves the use of a surface coil for irradiation and detection, placed on the surface of the sample as near as possible to the sample region which is to be examined. These unconventional coils are often used for whole animal and human NMR studies of metabolism where it is desirable to obtain in vivo spectra from individual tissues and organs lying on the surface or deep within the sample. The geometry of a surface coil determines that the excitation rf pulse angle decreases rapidly with penetration into the sample. For example, the pulse angle drops to one third at a depth of the radius of the surface coil. Clearly it is difficult to obtain definitive quantitative results with such a gross variation of pulse angle, and thus signal intensity, across the sample.
The various techniques of multipulse NMR depend on pulse angles being close to the ideal .pi./2 and .pi. values. When this is not so a wide variety of artifact or spurious signals are generated. Multipulse NMR using inhomogeneous radiofrequency coils would then seem to be impossible unless some means can be found to discriminate against sample regions where pulse angles diverge markedly from the ideal .pi./2 and .pi. values.
The prime object of this invention is to provide a means which will discriminate against some sample regions and select other sample regions, to the extent that the signal intensity in the experimental situation is vanishingly small from a large part of the normal sample volume, when using an inhomogeneous irradiation coil such as a surface coil.
It is a further object of this invention that it may be used for the application of various techniques of multipulse NMR using inhomogeneous irradiation coils.
The first object is accomplished in accordance with the invention by use of specific phase-alternated consecutive pulse schemes which require the pulse phases and receiver phase to be alternated in a prescribed way and which require the length of the pulses to be adjusted in a predetermined manner. The second object is accomplished by inserting delay periods in these pulse schemes in a prescribed manner.